1. You shouldnt have let them decorate, as you're not in control of the standard thats its done to
2. They've ****ed up. Tell them you'll do it all... plaster and redecorate. They'll lose their security deposit, but thats cos they're costing you a load of money. At the end of the day you might be a little out of pocket but you get piece of mind that its being done right.
3. Dont renew their lease.